The present invention relates to a control rod and a control rod blade for a boiling water reactor, wherein the control rod blade comprises a free edge portion with a recess which comprises a plurality of outlets, arranged in a row, for channels, which are arranged to receive an absorber material and a cover element arranged to be attached along at least a section of the edge portion, which cover element comprises a cover portion arranged to seal the opening of the recess and a support portion arranged to, in a mounted state, abut a wall surface in the recess and to allow the formation of at least one passage between the outlets of the channels in the recess.
Usually, control rods for boiling water reactors (BWR) comprises a central part and four control rod blades protruding from the central part, which each is provided at a right angle in relation to the adjacent control rod blades. Usually, conventional control rod blades are manufactured of a sheet of a steel material. The control rod blades are provided with free edge portion with a longitudinally milled recess a. A plurality of channels are drilled in the recess in a row above each other. The channels have a diameter, which is less than the thickness of the metal sheet such that a satisfactory wall thickness is obtained. The channels extend from the free edge portion of the control rod blade towards the central part of the control rods. A suitable absorber material is arranged in the channels. Thereafter, the absorber material is enclosed by rolling the legs of the recess together. Thereafter, the end surfaces of the legs are welded together with a longitudinal weld joint such a hermetic and pressure resistant sealing of the recess is obtained. A relatively deep recess must here be provided at the edge portion of the control rod blade such that the legs will obtain a length such that they can be rolled together. In a welded state, the free edge portion of the control rod blade here gets a rounded shape and consequently, the absorber material has to be arranged at a not completely negligible distance from the end surface of the control rod blade. In order to increase the reactivity effect of the control rod blade, it would be desirable if the absorber material could be arranged closer to the end surface.
By a profylax published in Research Disclosure 33925/92 a control rod blade is shown, which is provided with a concave recess at a free edge portion. In order to seal the recess, a cover element, which has a substantially corresponding shape, is here applied in the recess. The cover element comprises a cover portion, which seals the opening of the recess, and a support portion, which comprises a convex contact surface arranged to abut a corresponding concave wall surface of the recess. The cover element is fixed by means of two longitudinal weld joints to the legs of the recess. The support portion comprises a chamfered portion such that a passage is formed between the chamfered wall surface of the support portion and the concave wall surface of the recess, which passage connects the outlets of the channels in the space to each other. The channels here contain a powdered absorber material. The passage allows a flow of the fission gases, which are created during operation, between the channels such that a pressure equalization is obtained. The cover element has a shape, which allows the arrangement of two further passages. These further passages are arranged substantially immediately inside the weld joints. Passages allow distribution of a protective gas to the root side of the weld joints during the welding operation. The cover element has in this case a relatively complicated construction at the same time as a concave-shaped recess results in that the outlets of the channels, at least locally, are located at a relatively large distance from the end surface of the control rod blade. The possibility to arrange the absorber material near the end surface of the control rod blade is thereby reduced.